Temporarily plugging an earth formation with a transiently gelling aqueous liquid

ABSTRACT

A permeable earth formation is temporarily plugged by contacting it with a substantially homogeneous liquid mixture of an aqueous solution of a metal that precipitates as a gel-forming gelatinous metal hydroxide, a pH-increasing reactant that reacts at a selected rate to cause the precipitation, and a pH-decreasing reactant that reacts at a selected slower rate to subsequently dissolve the precipitate.

United States Patent Richardson 1 June 11, 1974 TEMPORARILY PLUGGING AN EARTH [56] References Cited FORMATION WITH A TRANSIENTLY UNITED STATES PATENTS GELLING AQUEQUS LIQUID 3,599,718 8/1971 Richardson et a1 166/292 75 Inventor; Edwin Allen Richardson Houston, 3,614,985 10/ 1971 Richardson 166/292 3,730,272 5/1973 Richardson et a1. 166/294 3,740,360 6/1973 Nimerick 166/293 X [73] Assignee: Shell Oil Company, Houston, Tex. 3,756,315 9/1973 Suman, Jr. et a1 166/294 X [22] Filed Apr 27 1973 3,766,984 10/1973 Nimerick 166/295 X [21] Appl. NO; 354,965 Primary Examiner-Stephen J. Novosad Related U-S. Application Data [60] g g'j g 32 A permeable earth formation is temporarily plugged 62 Au 1 l 1970 abandoned by contacting it with a substantially homogeneous liquid mixture of an aqueous solution of a metal that pre- [52] U S Cl 166 R81 l66/280 166092 cipitates as a gel-forming gelatinous metal hydroxide, 166/254 a pH-increasing reactant that reacts at a selected rate [5 I t0 021L186 the precipitatiOn, and a pHdeC1'6aSing reac- E2) 43/27 tant that reacts at a selected slower rate to subse- [58] Field of Search 166/281, 292, 294, 295, quemly dssolve the preclpltate- 10 Claims, 10 Drawing Figures OIL SAND 4 6 TH/EF ZONE 4 7 OIL SAND PATENTEDJUN 1 1 m4 (1815168 1 SHEET 10F 2 F/Gl PMENTEDJUH 1 1 m4 3.815681 sum 2 0F 2 A. AA

TEMPORARILY PLUGGING AN EARTH FORMATION WITH A TRANSIENTLY GELLING AQUEOUS LIQUID RELATED PATENT APPLICATION The present application is a continuationin-part of patent application Ser. No. 256,507 filed May 24, 1972, now abandoned which was a division of patent application Ser. No. 62,931 filed Aug. 11, 1970, (now abandoned).

It is also related to the E. A. Richardson US. Pat. No. 3,614,985, which describes an earth formation plugging liquid containing an aqueous solution of a metal that precipitates as a gelatinous metal hydroxide at a relatively high pH and a reactant that raises the pH of the aqueous solution to cause the precipitation. The present application relates to a relatively high pH and a reactant that raises the pH of the aqueous solution to cause the precipitation. The present application relates to a liquid in which such a metal and such a pH- increasing reactant are combined with a pH-decreasing reactant which lowers the pH of the aqueous solution and subsequently causes a dissolution of the precipitated metal hydroxide.

BACKGROUND OF THE INVENTION This invention relates to temporarily decreasing the permeability of a permeable material. More particularly, it relates to temporarily plugging a permeable earth formation by an in situ formation of a gel that subsequently reverts to a free-flowing liquid.

SUMMARY OF THE INVENTION The present invention realtes to temporarily plugging a permeable earth formation. A transiently gelling liquid made up to contain a substantially homogeneous mixture of an aqueous solution of a salt of a metal which precipitates as a gelatinous metal hydroxide at relatively high pH, a reactant that reacts at a selected rate to increase the pH of the aqueous solution and cause the precipitation, and a reactant that reacts at a relatively slower rate to subsequently lower the pH of the aqueous solution and cause the dissolving of the precipitated metal hydroxide. The compositions and proportions of the pH-increasing and pH- decreasing reactants are correlated so that, at a given temperature, the precipitation occurs within a selected time and forms a gel that tends to immobilize the solution, the gel breaks after a selected period, and the solution becomes relatively mobile. At least a portion of the earth formation to be plugged is permeated with the solution before the gel has formed. The solution is kept substantially static during the persistence of the gel and, after the breaking of the gel, is displaced from the earth formation.

DESCRIPTION OF THE DRAWING FIGS. 1 and 2 are schematic vertical sectional illustrations of two stages of a workover operation in a dual completion well.

FIGS. 3 through 5 are similar illustrations of three stages of a perforation cleaning operation.

FIGS. '6 through 8 are similar illustrations of three stages of a hydraulic fracturing operation.

FIGS. -9 and 10 are similar illustrations of two stages of a selective acidization of a relatively impermeable portion of a reservoir interval.

DESCRIPTION OF THE INVENTION The present invention is particularly useful in temporarily plugging subterranean earth formations into which wells have been completed. For example, in various well completion situations, a multiple zone well may be equipped with a casing string which is perforated in at least one upper and one lower reservoir interval. In various operations in a multiple zone well; it is desirable to vary the pressure of the fluid within the casing without causing fluid to flow into or out of one or more of the reservoir intervals. Such operations may involve fracturing the lower reservoir interval by pressurizing the fluid in the casing, cleaning the well by displacing sand or debris from below an upper reservoir interval by circulating fluid through the casing to a surface locations, or the like operations.

In other situations a well may be opened into a reservoir interval that contains layers having different permeabilities. In the latter situation it may be desirable to preferentially acidize or displace residual oil from the least permeable one of such layers. A preferential-treatment can be accomplished by injecting a transiently gelling fluid into the reservoir interval. In such an interval most of the injected fluid tends to enter and penetrate deepest into the most permeable layer. The soinjected transiently gelling fluid is allowed to gel. Subsequently, a treatment fluid is injected into the interval and is deflected, by the gel, into the least permeable layer. The gel later reverts to a free-flowing liquid.

The present transiently gelling fluids can also be used in well testing and various other operations. For example, in one type of injectivity profile well logging operation, a discrete slug of relatively cool liquid is injected into a relatively hot reservoir and measurements are made of the rate of the temperature recovery in order to determine the injectivity profile of the reservoir. A transiently gelling fluid of this invention can be advantageously used as the liquid which is injected. It is adapted to gel, and thus prevent the formation of convection currents during the temperature recovery. It is also adapted to subsequently revert to a free-flowing liquid. The present transiently gelling fluids can also be used to temporarily plug or stabilize various permeable materials, such as the sand of a foundry mold, or the like.

FIGS. 1 and 2 show a use of the invention in a workover in a well completed into upper and lower zones of permeable earth formation. Well borehole 1 contains cement sheath 2 and string of casing 3. The borehole is opened into fluid communication with upper and lower zones 5 and 6 of permeable earth formations by means of perforations 7 and 8 through the casing and cement. In FIG. 1, a tubing string 10 is equipped with a bottom cap 11 and a packer l2 and arranged for injecting fluid through the perforations 7. Transiently gelling fluid 13, in its initial liquid state is injected, as is indicated by arrows, into the region 14. A downflow within the borehole is prevented by packer l2 and an upflow is prevented by means of an additional packer (not shown) and/or by maintaining a back pressure on the annulus within casing 3, (i.e., the space between tubing 10 and easing 3).

. Transiently gelling fluid 13 is maintained static until it becomes gel 16 (FIG. 2) comprising a precipitated galatmous metal hydroxide. The gel is removed from the relatively large spaces within the borehole by a procedure such as a fluid circulation with an inflow through tubing and an outflow through the annular space between the tubing and the casing. Such a fluid circulation disrupts the structure of portions of the gel that are contained in the relatively large openings within the borehole, entrains the gel, and conveys the entrained gel to a surface location. This leaves the borehole open while the region 14, within the earth formation 5, is plugged by gelatmous metal hydroxide gel 16. In the relatively small pores within a permeable earth formation, the gel 16 is resistant to displacement in response to a relatively high pressure differential.

In the operational stage shown in FIG. 2, tubing'l0 has been replaced by a string of tubing 17 which is extended to near the perforations 8 leading into the lower zone 6 of permeable earth formation. As indicated by arrows, fluid may be circulated, with an inflow through tubing 17 and an outflow through the annulus between that tubing and casing 3, to entrain and remove solid particles 18, such as sand or debris. Such a fluid circulation is apt to involve fluctuations, in the injecting and- /or flowing pressures, which could cause undesirable inflows or outflows of fluid through the upper perforations 7. In using the present invention, such inflows or outflows are prevented by the presence of gel 16 in the zone 14 around the perforations. The gel 16 subsequently reverts to a free-flowing liquid that can be produced into the borehole or displaced further into the formation where it does not interfere with the operations of the well with respect to the upper zone.

FIGS. 3 through 5 show a use of the present invention in a procedure for cleaning each of a plurality of perforations that open into a reservoir interval and being certain that each of the perforations has been cleaned. A cased borehole 21 is opened into a reservoir 22 through perforations 23 and 24. Such a casing is preferably surrounded by a sheath of cement (not shown) with the perforations being extended through the casing and cement.

In FIG. 3, a string of tubing 26 surrounded by packer 27 has been installed with the packer set to plug the annular space between tubing 26 and casing 21. In the illustrated stage of the operation, the region 28 around perforation 23 and the region 29 around perforation 24 have been plugged with a gelatmous metal hydroxide gel 16.

During such a plugging operation, a slug of transiently gelling fluid is injected, at a downhole pressure greater than the reservoir pressure but less than the reservoir fracturing'pressure. Where one or a plurality of perforations (and/or the perforation tunnels extending through a cement sheath) is relatively highly permeable (e g., perforation 23 is cleaner and more permeable than perforation 24), most of the injected fluid flows through the more permeable perforation and permeates a relatively large zone (e.g., zone 28) around the clean perforation with only a small zone (e.g., zone 29) being permeated around the less permeable perforation.

In FIG. 4, gel 16 is being removed from the borehole. Tubing string 26, with packer 27 unseated, has been extended to near the bottom of the perforated interval. Fluid is circulated with an inflow through tubing 26 and outflow through the annulus around it, as shown by arrows. The circulating fluid breaks up the gel structure and entrains the broken-up portions of the gel 16 where the gel is unsupported in the relatively large openings existing within the borehole.

In FIG. 5, the packer and tubing string have been relocated and reset, in the positions shown in FIG. 3, and

a second slug of temporary gelling fluid 13 is being injected. During such an injectiomwhere a relatively large and relatively pressure resistant plugged zone 28 exists around one perforation and a much smaller plugged zone 29 (as shown in FIG. 3), exists around another, two events tend to occur. First, no fluid injection occurs at the injection pressure used in the prior injection of transiently gelling fluid. Second, after the injection pressure has been increased to a higher pressure (less than the fracturing pressure), the small plug in and around a debris plugged perforation (e.g., perforation 24) is displaced and fluid enters the surrounding zone 31. If substantially no fluid injection occurs at such an increased pressure, it is likely that (a) all perforations were clean and were plugged around during a previous injection of transiently gelling fluid or (b) the plugging material in any perforations which are still plugged is embedded too firmly to be removed without fracturing the reservoir. The gel 16 subsequently reverts to a free-flowing fluid that can readily be displaced when the well is returned to normal operation.

FIGS. 6 through 8 show a use of the invention in forming, extending and propping a fracture. In FIG. 6 a cased borehole 32 is opened through perforations 33 into reservoir 34 which has some, but a limited amount of, permeability. Such a reservoir may either be one which tends to fracture horizontally (as shown) or one which tends to fracture vertically. In the stage shown in FIG. 6, a fracture 35 has been formed, e.g., by injecting fluid through tubing 36 extended through packer 37, and is being held open by the pressure of a transiently gelling fluid 13 that is flowing into the fracture and the surrounding zone 38 as indicated by arrows.

In the stage shown in FIG. 7, the fluid injected into zone 38 has been kept stationary until it formed gel 16 and the gel that formed within the borehole has been removed. A fracturing fluid is being injected as shown by arrows to form a fracture extension 39 from which the fracturing fluid is leaking into the reservoir in the zone 41. Leakage through the fracture walls near the reservoir is prevented by the gel plug 16 in zone 38.

In the stage shown in FIG. 8, a granular fracture propping agent 42 is being pumped into the fracture as shown by arrows. The fracture propping material suspending liquid leaks into' the zone 41 and displaces fluid from that zone into the expanded zone 43. Leakage near the well is prevented by the gel 16 in the zone 38. The gel facilitates the transporting of fracture propping material for a relatively long distance within the fracture by preventing the leakage into the fracture walls that tends to cause sand-outs or depositions of the fracture propping material. The gel 16 in the zone 38 subsequently reverts to a free-flowing fluid that is readily displaced when the well is used for the injection or production of fluid.

FIGS. 9 and 10 show the use of the invention in a process for selectively acidizing a layer of lesser permeability within a reservoir interval that contains layers of different permeabilities. As shown in FIG. 9, the well contains a perforated casing 44,'a tubing string 45, and has been pretreated by injecting a transiently gelling liquid and allowing it to form a gel 16 within'the portions 48 and 49 of the reservoir formation. The reservoir interval includes oil sand layers 46 and thief zone layer 47. Since the oil sands are less permeable than the thief zone, they are permeated and plugged only to the depths 48, which are small relative to the depth 49 of the plugging in thief zone 47. l

In the stage shown in FIG. 10, an acidizing fluid 51 is being injected as shown by the arrows. Such an acidizing fluid can be substantially any relatively strong mineral and/or organic acid formulation, such as the acidizing fluids conventionally used for increasing the permeability of an earth formation. The acidizing fluid can be prevented from rising within the annulus between tubing 45 and casing 44 by maintaining back pressure by means of surface located equipment or downhole packers (not shown). The pressure at which the acidizing fluid is injected exceeds that used to inflow the transiently gelling fluid but is preferably less than the reservoir-fracturin g pressure. A so-pressurized injection of fluid tends to displace portions of the relatively thin plugs in zone 48 without displacing the relatively thick plug in zone 49. As acidizing fluid 51 flows through the gelled metal hydroxide in zones 48, it tends to dissolve the plugging material and most of the acidizing fluid tends to be guided around the undisturbed plug in zone 49 into the relatively impermeable oil zones, as indicated by the arrows. The undisplaced and undissolved portions of the'gel 16 subsequently revert to a free-flowing liquid.

The transiently gelling fluid of the present invention I can comprise substantially any relatively homogeneous liquid mixture that is adapted to penetrate into the pores of a permeable earth formation and contains an aqueous solution of at least one multivalent metal that precipitates as a gelatmous metal hydroxide at relatively high pH, at least one pH-increasing reactant which reacts at a relatively rapid rate to form at least one alkaline material that increases the pH of the aqueous solution and at least one pH-decreasing reactant that reacts more slowly to form at least one water soluble acidic material that decreases the pH of the aqueous solution. The transiently gelling fluid can be a clear solution, a micellar dispersion, or an emulsion containing a dispersed phase in which the particles are sufficiently small and mobile to move through the interstices within a permeable earth formation.

The aqueous liquid solution of a metal which precipitates at relatively high pH in the form of gelatmous metal hydroxide can comprise a solution of substantially any salt of such metal which is soluble in an aqueous liquid having a moderately low pH and is precipitated from an aqueous solution of a moderately higher pH. Such salts are preferably soluble in a pH range of from about 2 to 7 and precipitated at a pH from about 7 to 10. The preferred salts are typified by the chloacid halides, N-alkylamides, urea, salts of cyanic acid, such as alkali metal cyanates, cyanamide, etc. Urea and potassium cyanate are particularly suitable reactants that increase the pH of an aqueous solution by producing ammonium hydroxide at rates which are suitable for use at temperatures commonly encountered in subterranean earth formations.

Where desirable, a buffer system, such as a dissolved mixture of a weak acid and its salt of a strong base, can be used to facilitate and/or control the adjustment of the pH of the solution. For example, a substantially equal molar mixture of acetic acid and sodium acetate will maintain a pH of from about 4.5 to 5.0 as long as both components are present. Such a mixture can be used to extend the time required for the ammonia released by the hydrolysis of urea-to raise the pH of the solution. The first portions of the ammonia are used up in depleting the components of the buffer and the attainment of the pH at which the metal hydroxide is precipitated is delayed by the time required to overcome this effect.

The pH-decreasing reactant can be substantially any relatively slowly reacting material (preferably a watersoluble material) which yields a watersoluble acidic product that decreases the pH of an aqueous liquid. The relative amounts and rates of each of the pH- increasing and pH-decreasing reactants are adjusted by coordinating the types and amounts of such reactants so that the aqueous solution first gels and becomes immobile, as the pH increases to a value at which a gelatmous metal hydroxide is precipitated, and then breaks and becomes a mobile fluid, as the pH decreases and the metal hydroxide is dissolved. The concentration of the respective metal salt and pH-increasing and pH- decreasing reactants can be varied over relatively wide limits. Such concentrations can range from as little as about 0.1 percent by weight to an amount (which may be as high as from about 30 to percent) that forms a saturated solution. Suitable pH-decreasing reactants are exemplified by the'hydrolytically-reactive halogenated organic compounds described in the solvolysis acidization process U.S. Pat. Nos. 3,215,199; 3,297,090; and 3,307,630; esters such as esters of water-soluble organic or inorganic acids, such as formic acid, acetic acid, chloroacetic acid, acids such as phosphoric, sulfuric, nitric, etc., with water-soluble alchols, such as methyl, ethyl, propyl, or the like reactants.

Relative to the plug formation and plug dissolution reactions the pH-triggered transiently gelling fluids of the present invention are uniquely advantageous. During the plug formation, the viscosity and flow properties of the fluid remain substantially unchanged while the pH is slowly increasing. When the precipitation inducing pH is reached, substantially all of the dissolved metal becomes insoluble in substantially all of the aqueous liquid. The change from a free-flowing liquid to an immobile gel is relatively sudden. During the plug dissolution, the rigidity and insolubility of the gel remain substantially unchanged while thhe pH of the aqueous liquid component is slowly decreasing. When the metal dissolving pH is reached, substantially all of the precipitated metal hydroxide become soluble in substantially all of the aqueous liquid component. The change from an immobile gel to a free-flowing liquid is both relatively sudden and complete.

The transiently gelling liquids of the present invention are characterized by certain readily determinable properties. These properties can be evaluated from test tubes by visual observation. In general, a test solution is placed in a test tube, sealed and immersed in a waterbath. Time is taken as zero at the moment'of immersion in the bath, without allowance for warm-up At intervals the contents of the tube and the following observations are made.

1. IPT (Initial precipitation time). A transiently gelling fluid of the present invention is usually a clear, water-white solution, having a density and viscosity ap proximating that of fresh water. At the time its components are mixed in the laboratory or blended in a field operation, such a solutionhas a pH of from about 3 to 6. The reaction of typical pH-increasing reactants, such as the hydrolysis of urea and KCNO, cause the pH to rise until a gel forms, at a pH of about 6.5. The time, at a given temperature required to form the gel is referred to as IPT. The gel usually forms relatively quickly. The solution usually remains clear until the gel forms. The pH of the gelled liquid continues to rise until the pH-increasing reactant is spent. 2. GET (Gel Break Time). When the pH-increasing reactant is essentially spent, the rate of its production of base becomes about equal to the rate of production of acid by the pH-decreasing reactant, such as a hydrolyzing ester or organic cloride. The pH of the liquid thus becomes steady and eventually begins to drop as the rate of the base production continues to diminish due to the continued depletion of pl-l-increasing reactant. At a pH of about 6.5, the gel begins to break as a large unit, even though many large patches of gel may still be present as well as some individual suspended solid particles.

The Time at a given temperature required for the gel to start breaking is referred to as GBT. Thus, the GET contains the lPT plus a period of time during which the solution or precipitate-containing liquid was firmly EXAMPLE 1 An aqueous solution containing about 2 grams of KOCN per 100 cc is blended with an equal amount of an aqueous solution containing about 4.4] gram per 100 cc of AlCl,-, 6 H and 29.4 cc per lOO cc ofa 3% aqueous solution of ammonia. A 100 cc portion of that blend was mixed with 6 cc of ethylformate and 2 cc of an aqueous 3% ammonia. This yielded a transiently gelling solution having a pH of about 5.7 and containing a ratio of equivalents of pH-increasing reactant to pH- decreasing reactant of about 0.3. At about 140F, this solution exhibits an IPT of 5 minutes, a GBT of 3 hours, and a CT of 6 hours (at which the pH is 4.5).

EXAMPLE 2 A transiently gelling solution was prepared as described in Example 1, except that a 100 cc-portion of the blend of pH -increasing reactant and metal salt solutions was mixed with 4 cc of ethylformate, 2 cc of ethyl acetate, and 4 cc of a 3.7 percent hydrochloric acid. This yielded a transiently gelling solution having a pH of about 4.9. at 140F, this solution exhibited an IPT of 20 minutes, a GBT of two hours, and a CT of 8 hours (at which time a faint haze was apparent).

EXAMPLE 3 A transiently gelling solution was prepared substantially as described in Example 1 except that the KOCN reactant was replaced by urea and the blend of the pH- increasing reactant and metal-containing solutions was mixed with ethyl alcohol. At about 190F. the resultant transiently gelling solution exhibited an IPT of 2 hours, a GBT of 2 days, and a CT of greater than about 6 days (relative to a solution containing only a faint haze).

EXAMPLES 4 AND 5 An oil-in-water emulsion containing, respectively, about 2 cc of allyclchloride, 0.3 cc anionic surfactant (Triton GR-7, available from Rohm and Haas), and 1 gram KOCN per cc of water, was blended with an equal amount of a metal salt containing aqueous solution of the type described in Example 1. The blending provided a transiently gelling liquid of the present invention in the form of an emulsion. The results of testing that liquid at two different temperatures are indicated in the Table l: i

The results of measuring the viscosity as a function of time during the course of a gel-break cycle, in order to illustrate the type of changes occuring in a transiently gelling liquid of the present invention, are indicated in Table II. Since such a gelled system is highly non-Newtonian, the viscosity values are a function of arbitrary conditions of measurement and the numerical values have only a qualitative significance. An aqueous solution containing about 1 gram KOCN per 100 cc was blended with a metal salt-containing solution as described in Example 1. A 100 cc portion of the blend was mixed with 5 cc methylacetate and 0.2cc aqueous 3% ammonia. This yielded a transiently gelling solution having a pH of about 5.3. The viscosity, with time, at F, is indicated in Table II:

TABLE ll-Continued dures for compounding and using such mixtures are disclosed in the E. A. Richardson, R. F. Scheuerman patent application Ser. No. 144,260 filed May 17,

T, Time Viscosity at F! minutes Viscosity at pH Notes 1971; and those d1sclosures are 1ncorporated herem by 395 7.1; good gel reference. 500 7.2 good gel 680 6.6 good gel Referrmg to Table IV, aqueous solut1ons of alum1- E88 (1 day) g gg f GBT num chloride, sodium hydroxide, sodium or potassium 2:000 nitrite and urea were blended and mixed with 2- 2.600 (2 y 2.3 smokey 1O chloroethanol, substantially as described in Example 1. 1233 812x 3 {:22 The proportions used and the results of observing the 5.500 (4 days) 1.29 smokey WT and CT times at the indicated temperatures are 3 288 22 1'8; CT listed in the Table. It will be apparent that the relative 101100 7 days) colloidal proportions of the reactants can be varied by significant amounts and such variations can be utilized to ob- From the above data it is obvious that a sudden rise tain gel times and breakback times that are delayed by in viscosity occurs between and minutes after imselected amounts.

TABLE IV Example Al*"M "NaOH RNOQ/RV "CHZCICHZOH Temperature lPT (Hours) CT (Days) 7 0.5 2.195 114/114 2 170 3d 9 21 do. 2.17 do. 3 170 45 4.7 9 do. 2.l8 1.57s/1.105 3 do. 36 4.1 10 do. do. do. 4 do. 45 3 ll do. do. do. 2 do. 38 l 1.5 12 do. do. do. 4 180 32 2.2 13 do. do. do. 3.0 120 32 2.6 14 do. do. do. 180 27.5 3.7 15 do. do. do. 2.0 180 29.5 8" 16 0.6 do. 40 170 20 3.3 l7 do. do. do. 20.8 5.0 18 do. do. 25 do. 19.5 5.2

AI' M: molar concentration of aluminum ions (from dissolved aluminum chloride) "NaOH. "N0 v "L' and "CH,CICH.,OH; ratio of moles per mole of aluminum ions of, respectively, sodium hydroxide, nitrite ions, urea, and Z-chloroethanol.

About Ill; breakback in 8 days (when the experiment was discontinued) m ersion in the bath. At 21 minutes a good gel was noted, giving the indicated lPT value. The solution remains at a high viscosity until about 1,200 to 1,700 minutes has passed, giving a GBT of 1,700 minutes. A slow drop continues until t= 6,900 minutes, when essentially the original viscosity is reached. However, the pH does not go low enough to give a clear solution as dispersed colloidal solids too fine to settle are noted in the solution. It is not expected that particles this fine will cause formation damage in a subterranean reservo1r.

From the data presented above (and other data not given), a tentative pH table may be set up as follows:

TABLE 111 pH Range Solution less than 6.5 and increasing clear (low viscosity) greater than 6.5 and increasing gel forms (viscosity increase) gel begins to break (viscosity still to high) less than 6.5 and decreasing solution is smokey-to-hazy-to (1.5 and decreasing colloidal (low viscosity) less than 5.0 and decreasing clear (low viscosity) EXAMPLES 7 -18 In general, a transiently gelling fluid of this invention (as formulated for use in a subterranean earth formation having a temperature commonly encountered in such formation) is exemplified by an aqueous liquid solution containing from about 0.1 to 50% by weight of each of: aluminum chloride, urea, and ethyl acetate. The relative proportions are preferably such that the amount of urea is at least the stoichiometric equivalent for increasing the pH of the aqueous solution of the aluminum chloride to a pH at which aluminum hydroxide is precipitated, and the amount of ethyl acetate is at least the stoichiometric equivalent for reducing the pH of an aqueous liquid containing the products of reaction of the aluminum chloride and urea to a pH at which the precipitated aluminum hydroxide is redissolved.

What is claimed is l. A process for temporarily plugging a permeable earth formation, which comprises:

compounding an aqueous liquid solution, of from about 0.1% by weight of a saturated solution of each of (a) at least one salt of a polyvalent metal that is soluble in an aqueous liquid of pH from about 2 to 7 and precipitates as a gelatinous metal hydroxide when the pH of the aqueous liquid is increased to from about 7 to 10 (b) at least one water-soluble, water-reactive reactant which increases the pH of the aqueous liquid and causes said precipitation and (c) at least one watersoluble, relatively slowly water-reactive reactant which yields a water-soluble acidic product that decreases the pH of the aqueous liquid and causes the dissolution of the precipitated metal hydroxide;

adjusting the relative amounts and rates of each of said pl-l-increasing and pH-decreasing reactants and reactions by coordinating the types and amounts of said'reactants so that the aqueous solution first gels and becomes immobile, as the pH increases .to a value at which a gelatinous metal hydroxide is precipitated, and then breaks and becomes a mobile fluid, a the pH decreases and the precipitate is dissolved;

permeating an earth formation with said aqueous solution before the occurrence of said precipitation;

maintaining the'fluid substantially static within the earth formation until the precipitation occurs; and k after the occurrence of said dissolution of precipitated metal hydroxide, displacing fluid from the sotreated earth formation.

2. The process of claim 1 in which:

the permeated earth formation is an upper zone of a multiple zone well, and

fluid is circulated within the well'while said upper zone is plugged with the precipitated metal hydroxide.

3. The process of claim 1 in which:

the permeated earth formation is the formation adjacent to at least the most permeable one of a series of substantially adjacent perforations through the casing of a well,

after said precipitation of metal hydroxide, but before said dossolution of metal hydroxide, the precipitated hydroxide is displaced from the perforated portion of the casing, and

at least one additional portion of said transiently gelling fluid is injected into the earth formation adjacent to any of said perforations through which such fluid will flow in response to a pressure that exceeds the pressure at which the preceding portion of said fluid was injected but is less than the fracturing pressure of the surrounding earth formations.

4. The process of claim 1 in which:

the permeated earth formation is one that forms the wall of a fracture, and

said fracture is extended and propped while its walls are plugged with said precipitated metal hydroxide.

5. The process of claim 1 in which:

the permeated earth formation is the most permeable portion of a reservoir interval containing'formations of different permeabilities, and

a treating fluid is injected into said reservoir interval while the most permeable portion is plugged'with the precipitated metal hydroxide.

6. A process for temporarily plugging a permeable earth formation which comprises:

compounding a substantially homogeneous liquid consisting essentially of an aqueous solution of a salt of a metal that precipitates as a gelatinous metal hydroxide when the pH of the solution is increased, a pH-increasing reactant that reacts at a selected rate to increase the pH of the aqueous solution and cause the precipitation, and a pH- decreasing reactant that reacts slower than the pH- increasing reactant to subsequently decrease the pH of the aqueous solution and cause a dissolution of the precipitated metal hydroxide;

adjusting the compositions and proportions of said pH-increasing and pH-decreasing reactants so that, at a given temperature, the precipitation occurs within a selected time and forms a gel that substantially immobilizes the liquid, the gel breaks after a selected time, and the liquid becomes relatively mobile;

permeating at least a portion of the earth formation to be plugged with the liquid before the gel has formed;

maintaining the liquid substantially static during the persistence of the gel; and

after the breaking of the gel, displacing the liquid from the earth formation.

7. The process of claim 6 in which a buffering agent is dissolved in said aqueous solution.

8. The process of claim '6 in which said pH-increasing reactant is awater-soluble material that reacts with water to yield an amino group-containing material.

9. The process of claim 6 in which said pH- decreasing reactant is a water-soluble ester that reacts with water to yield a water-soluble acid.

10. The process of claim 6 in which the pH- increasing reactant is a mixture of urea and an alkali metal nitrite. 

2. The process of claim 1 in which: the permeated earth formation is an upper zone of a multiple zone well, and fluid is circulated within the well while said upper zone is plugged with the precipitated metal hydroxide.
 3. The process of claim 1 in which: the permeated earth formation is the formation adjacent to at least the most permeable one of a series of substantially adjacent perforations through the casing of a well, after said precipitation of metal hydroxide, but before said dossolution of metal hydroxide, the precipitated hydroxide is displaced from the perforated portion of the casing, and at least one additional portion of said transiently gelling fluid is injected into the earth formation adjacent to any of said perforations through which such fluid will flow in response to a pressure that exceeds the pressure at which the preceding portion of said fluid was injected but is less than the fracturing pressure of the surrounding earth formations.
 4. The process of claim 1 in which: the peRmeated earth formation is one that forms the wall of a fracture, and said fracture is extended and propped while its walls are plugged with said precipitated metal hydroxide.
 5. The process of claim 1 in which: the permeated earth formation is the most permeable portion of a reservoir interval containing formations of different permeabilities, and a treating fluid is injected into said reservoir interval while the most permeable portion is plugged with the precipitated metal hydroxide.
 6. A process for temporarily plugging a permeable earth formation which comprises: compounding a substantially homogeneous liquid consisting essentially of an aqueous solution of a salt of a metal that precipitates as a gelatinous metal hydroxide when the pH of the solution is increased, a pH-increasing reactant that reacts at a selected rate to increase the pH of the aqueous solution and cause the precipitation, and a pH-decreasing reactant that reacts slower than the pH-increasing reactant to subsequently decrease the pH of the aqueous solution and cause a dissolution of the precipitated metal hydroxide; adjusting the compositions and proportions of said pH-increasing and pH-decreasing reactants so that, at a given temperature, the precipitation occurs within a selected time and forms a gel that substantially immobilizes the liquid, the gel breaks after a selected time, and the liquid becomes relatively mobile; permeating at least a portion of the earth formation to be plugged with the liquid before the gel has formed; maintaining the liquid substantially static during the persistence of the gel; and after the breaking of the gel, displacing the liquid from the earth formation.
 7. The process of claim 6 in which a buffering agent is dissolved in said aqueous solution.
 8. The process of claim 6 in which said pH-increasing reactant is a water-soluble material that reacts with water to yield an amino group-containing material.
 9. The process of claim 6 in which said pH-decreasing reactant is a water-soluble ester that reacts with water to yield a water-soluble acid.
 10. The process of claim 6 in which the pH-increasing reactant is a mixture of urea and an alkali metal nitrite. 